A bulge is the collective designation for tightly packed stars of a comparable age within the core of a galaxy, just as a slaughter of crows, apes’ shrewdness, and a murmuration of starlings.
The cores of most galaxies feature bulges. The bulges are given different names according to their properties—specifically, the kinematics of their stars. Classical bulges move randomly, similar to elliptical galaxies, and appear to be older than their galaxy, whereas pseudobulges move by rotation, similar to spiral galaxies, and appear to be the same age as their galaxy.
How Did Astronomers Observe These Bulges?
To investigate these bulges, astronomers utilize the star halo of a galaxy as a “fossil record.” Astronomers can use stellar halos to determine whether a galaxy has already merged with another galaxy.
Despite having the greatest pseudobulge in the local universe, a University of Michigan doctorate student analyzed the pseudobulge of the nearby disc galaxy Messier 94 and discovered that no huge galaxies had likely crashed into Messier 94 in the past.
“Astronomers believe that when a galaxy merges with another galaxy, material is deposited in the stellar halo of the merging galaxy,” lead author Katya Gozman stated.
“We can research and learn about the past mergers of a galaxy by investigating and learning about the stars and stellar populations in the stellar halo. In the star halo around Messier 94, you could say we’re undertaking extragalactic archaeology.”
What Were The Observations Of The Messier 94’s Stellar Halo?
Gozman examined Messier 94’s stellar halo, which is the diffuse halo of stars that surrounds a galaxy, using data from the Subaru Hyper Suprime-Cam in Hawaii. The stellar halo stretches far beyond what a galaxy appears to be at first glance, and astronomers can study it for relics of previous mergers. Calculating the mass of a galaxy’s halo is one technique to learn about these remains.
Gozman used Subaru data to categorize stars in M94’s stellar halo based on their brightness for this research. She utilized a color magnitude diagram to classify stars based on their brightness as seen through various filters used in astronomy to determine how much light they produce.
What Are RGB Stars?
She was looking at a red-giant branch star, or RGB star, in particular. These stars are bright, which is advantageous for imaging, according to Gozman, and the amount of heavier, metallic elements they contain is highly correlated with their color.
The RGBs in the galaxy were then separated into two groups: RGBs with more blue light and RGBs with more red light, according to Gozman. The blue RGBs had a low metal content, whereas the red RGBs had a higher metal content. She also mapped the distribution of stars in the galaxy, discovering that the red RGBs are concentrated in a ring around the galaxy’s center, while the blue RGBs are distributed throughout its halo.
The Shapes Of The Galaxies
Gozman divided the galaxy into circular annuli, or concentric rings, covering the disc like a bullseye, focusing on the blue RGBs. She was able to determine the mass of the stellar halo—which was not at all massive—by estimating the surface brightness of the stars in each ring.
The mass of the halo allows us to calculate the mass of the galaxy that merged into it.
“We deduce the mass of the galaxy that last slammed into the galaxy we’re looking at based on the mass of the halo,” Gozman explained. “One could suppose that if a massive galaxy collided with M94 a long time ago, it would have dramatically affected the galaxy’s morphology, components, and possibly given origin to this massive pseudobulge in the center.”
However, Gozman discovered that there was no big merger. In the past, the largest galaxy that collided with M94 was not very enormous. Instead, she believes the pseudobulge originated as a result of the galaxy’s normal evolution.
“This is the first time we’ve ever had data on the galaxy’s resolved star population. Resolving stars is a difficult task, but it is one of the most effective ways to study the halos and learn about the galaxy’s merger history “she stated “So this is again another data point in a field with very few.”